Clean up redundancy within radio component

[danacotoran]

Recently the ability to pass a custom heading level was introduced to the radio component #1951

Since we've now got this option, the is_page_heading option is a bit redundant/confusing.
The is_page_heading parameter simply dictates that the heading be a h1 of size xl.
It is possible to retire is_page_heading and use heading_level and heading_size to achieve the same result.